The general objective of this research has been to enhance our understanding of the influence of the autonomic nervous system on electrophysiologic properties of the heart. We have focused our attention on two important areas of the specialized conduction system; namely, distal Purkinje fibers and the sinoatrial node. In both groups of studies observations have been made in the intact heart as well as in isolated tissues removed from the heart and studied with microelectrode techniques. We have demonstrated that at low external potassium concentrations propranolol has a hyperpolarizing effect on Purkinje fibers. We are currently investigating the effects of propranolol on the input resistance to Purkinje fibers at low potassium concentrations. These studies are designed to provide an explanation for the mechanism of action of propranolol on the Purkinje fibers and the basis for understanding its antiarrhythmic influence. A second group of studies have been directed at the analysis of sinus node responses to premature stimulation. Comparable techniques have been employed in the intact heart and in the isolated right atrial tissue containing the sinus node. These studies have provided a basis for understanding the influence of premature stimuli on the cycle length of sinus node activity.